When you say "but sometimes struggle with fast playing once i have played a piece to much." - I believe this is saying something about the learning process and what stage you are at with the piece in question and/or some connection with the level of your piano technique. I have these suggestions:
1: Playing separate hands, in this case the problem is often the RH - play the entire passage in alternately syncopated fashion, ie: Da, di-Da, di-Da etc. Then Da-di, Da-di, Da-di etc. Start slow and very precise, then faster and constantly repeating. When you get the hang of it then alternate the two types and also randomly decide to vary the times you play the one syncopation or the other. Sometimes deliberately vary the accenting. When you can freely and without inner arwkward compulsion - play both ways and at a reasonalbe turn of speed, then you will often find that you can then play the passage "straight" and evenly and fast.
Try playing the offending passages in the highest register, co-ordination will then be easier lower down.
If you have access to a computer or are just handy with a pencil, write or print out the passages in a few different keys and play them.
If the problem happens only at set places, often to do with slightly more difficult note/finger positions like issues around black/white notes, then isolate the few notes and make it a technical exercise and move up and down a few semitones etc. If necessary divide the uneven part down to just two notes or three and treat each as an exercise, and progressively move through the passage.
Play the passage with more flattened fingers legato, then staccato, then with fingers curved like hammers, staccato then legato. Find the best finger profile and works the best. Try putting an "impulse on each note, then try treating the groups as one phrase"
For every time you play the piece at full speed, play it ten times at a slower speed.
But not only this, play the piece Largo and with feeling and rubato (tempo), and different kinds and places of crescendo etc etc , be creative with it, sing with it.
Play it on a different piano (This can make a big difference, heavier or ligher touch, what you have may not suit your playing style)
Develop a constant background of scales and appeggios to get your fingers used to more rapid playing as a baseline.
Work *very* hard at concentrating on making your fingers stay even.
Put the other hand in a computer sequencer, or have a friend play the other hand, and play along, try playing with no pedal at all, if the uneveness is to do with the thumb or 4th and 5th fingers, practice motiffs all over the piano as a standard daily exercise.
Seek out and talk to as many piano teachers as you can and address to them the specific problem in context with the music, if possible get them to demonstrate what they do to address this vexing issue.
Try playing immediately after sleep when no other thing has occupied your mind. Go straight to the piano be creative and think and try the things above outlined. DO NOT play it at a speed that your fingers become uneven. When the warning does come, then buy yourself some flowers and laugh. Play more expressively . A piece can sound fulfulling, even fast, if you create the illusion of it, try at times a very exaggrated rubato and vastly different kinds of sound dynamics, make it whisper, then shout, but not always in the same places or the same way!. Often, play with your eyes closed, other times look at your hands as you play, from different angles.
Try a lower and then a higher chair.
I am sure your speed, and any uneveness will begin to dissolve when you work through a few of the above suggestions:-)
Do all these things and anything that you find works for you, to develop more consciousness of what you are doing.
All the best.
Topic: The Fantasie Impromptu and fast playing